In plasma processing systems, RF (radio frequency) energy is transmitted into a reaction chamber by an RF power supply, to excite reaction gas in the reaction chamber to generate plasma containing a large number of active particles, and the plasma interacts with a wafer, thereby completing a process such as etching or deposition. In transmission of the RF energy, an output impedance of the RF power supply is typically 50 ohms, while an input impedance of the reaction chamber is not equal to 50 ohms. For this reason, in a case where the RF energy is directly transmitted into the reaction chamber, the RF energy may be reflected due to an impedance mismatching of the transmission path, causing that the reaction gas in the reaction chamber cannot be excited normally to generate the plasma. As such, a matching device connected between the RF power supply and the reaction chamber is required to ensure the normal transmission of the RF energy.
FIG. 1 is a functional block diagram of an existing matching device according to some embodiments of the present disclosure. Referring to FIG. 1, an RF system, which is configured to supply RF energy to a reaction chamber 60, includes an RF power supply 10 and a matching device 30. The RF power supply 10 has a pulse function. The matching device 30 is connected between the RF power supply 10 and the reaction chamber 60 and has an auto-matching function and a matching-position holding function. Specifically, the matching device 30 includes a detection unit 1, an impedance adjustment unit 2, a control unit 3, and two control motors 4 and 5. The detection unit 1 is configured to detect a signal on a transmission line at a front end of the impedance adjustment unit 2, and to transmit the signal to the control unit 3. The impedance adjustment unit 2 has an internal structure as illustrated in FIG. 2, in which two variable capacitors C2 and C3, a fixed capacitor C1, and fixed inductors L and L2 are included. The two control motors 4 and 5 are configured to respectively adjust the two variable capacitors C2 and C3 under the control of the control unit 3. At the beginning of a process, the RF power supply 10 is first switched into a continuous wave mode for outputting continuous wave power, and at the same time, the detection unit 1 transmits the input signal and the value of reflected power, which are detected in real time, to the control unit 3. The control unit 3 obtains adjustment amounts of the variable capacitors C2 and C3 based on a preset algorithm, and controls the two control motors 4 and 5 to adjust the two variable capacitors C2 and C3, respectively. In the adjustment process, the control unit 3 determines whether the value of reflected power, which is transmitted from the detection unit 1, is within a small threshold range. In a case where the value of reflected power is within the small threshold range, it is determined that the output impedance of the RF power supply 10 matches the input impedance of the matching device 30, and the impedance adjustment unit 2 is controlled to be in a hold mode in which the capacitance and other parameters of the two variable capacitors C2 and C3 are all kept unchanged, when the plasma remains stable. Meanwhile, the control unit 3 controls the RF power supply 10 to switch into a pulse wave mode for the process. After the process proceeds to a certain stage, the control unit 3 determines whether it is required to switch to a next process. If it is not required, the process continues until its end. If it is required, due to changes of all processing conditions, it is necessary to control the RF power supply 10 to switch into the continuous wave mode again and the matching device 30 performs the matching again. After completion of the matching, the impedance adjustment unit 2 is in the hold mode again, while the RF power supply 10 is switched into the pulse wave mode again for a new process. Rest processes are performed in a similar fashion until completion of all processes.
In actual applications, the above-described matching device 30 inevitably has the following problems.
First, an overshoot phenomenon may occur when the RF power supply 10 outputs the pulse power, causing a sudden change of the impedance of the RF system. Since the impedance adjustment unit 2 is in the hold mode while the RF power supply 10 is in the pulse wave mode, a response to the sudden change of the impedance cannot be made in time, so the impedance matching cannot be achieved continuously, which in turn affects the outcome of the process.
Second, a matching position of the impedance adjustment unit 2 in the continuous wave mode of the RF power supply 10 and that of the impedance adjustment unit 2 in the pulse wave mode of the RF power supply 10 are thought to be identical in default. In actual applications, however, the impedance of the RF system may be varied between these two modes under the same process conditions. As such, in a case where the matching positions are set to be identical in these two modes, the matching precision between the output impedance of the RF power supply 10 and the input impedance of the matching device 30 may be affected and the output impedance of the RF power supply 10 and the input impedance of the matching device 30 may even be mismatched. As a result, problems such as unstable matching and non-repeatable matching may occur, thereby affecting the outcome of the process.